Memory device with a sense amplifier

ABSTRACT

A circuit for a sense amplifier (14) for use with a memory device (10). The circuit includes two devices (40 and 42) that are controlled by a selector (44). The first device (40) drives the sense amplifier (14) with a first current level. The second device (42) drives the sense amplifier (14) with a second current level, different from the first current level. The selector (44) is coupled to the first and second devices (40 and 42) so as to selectively couple one of the first and second devices (40 and 42) to the sense amplifier (14) based on a power supply voltage of the memory device (10).

This application is a divisional of U.S. Ser. No. 08/915,271 filed Aug.22, 1997.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to electronic circuits and inparticular to a memory device with a sense amplifier.

BACKGROUND OF THE INVENTION

In the electronics industry, device manufacturers design and producecommodity parts that are capable of operating in a variety of electronicsystems that have different electronic specifications. For example,dynamic random access memory (DRAM) devices are designed for use inelectronic systems with a wide range of power supply voltage. To keeppace with changes in system specifications, device manufacturers facethe difficult task of designing new parts that operate over a widerrange of conditions.

A DRAM device is comprised of an array of individual memory cells.Typically, each memory cell comprises a capacitor capable of holding acharge and an access transistor for accessing the capacitor charge. Thecharge is representative of a data bit and can be either a high voltageor a low voltage. Data can be either stored in the memory cells during awrite mode, or data may be retrieved from the memory cells during a readmode. The data is transmitted on signal lines, referred to as digitlines, which are coupled to input/output lines through transistors usedas switching devices. For each bit of data stored, its true logic stateis available on an I/O line and its complementary logic state isavailable on an I/O complement line. Thus, each memory cell has twodigit lines, digit and digit complement.

Typically, the memory cells are arranged in an array and each cell hasan address identifying its location in the array. The array comprises aconfiguration of intersecting rows and a memory cell is associated witheach intersection. In order to read from or write to a cell, theparticular cell in question must be selected, or addressed. The addressfor the selected cell is represented by input signals to a row decoderand to a column decoder. The row decoder activates a word line inresponse to the row address. The selected word line activates the accesstransistors for each of the memory cells in communication with theselected word line The column decoder selects a digit line pair inresponse to the column address. For a read operation the selected wordline activates the access transistors for a given row address, and datais latched to the digit line pairs.

Conventional dynamic memory devices use memory cells fabricated ascapacitors in an integrated circuit to store data. That is, a logical"1" is stored as a charge on the capacitor and the capacitor isdischarged for a logical "0". The pairs of digit lines are fabricated asmetal lines on the integrated circuit and connected to the memory cellsfor transmitting data stored in the memory cells. Sense amplifiers areutilized to sense small differentials on the digit lines and drive thedigit lines to full power supply rails for either reading the memorycells or writing thereto.

Typically, a sense amplifier includes a pair of n-channel transistorshaving a cross-coupled gate and drain configuration. Due to the positivefeedback of this configuration, the sense amplifier senses slightchanges in the voltages on the digit and digit complement lines andproduces full logic values on the digit lines based on the slightvoltage differential. The source of each transistor is coupled to a pulldown device, which, in operation drives the source of the transistors toground thus allowing the sense amplifier to amplify the small changes involtage on the digit and digit complement lines.

Conventionally, the pull down device of an n-channel sense amplifiercomprises an n-channel MOS transistor. Unfortunately, conventional pulldown devices do not function properly over the wider range of powersupply voltages demanded by newer systems. At low supply voltages, thecurrent in a typical pull down device is not sufficient to allow thesense amplifier to settle quick enough to produce an accurate reading atthe digit lines. Further, at high power supply voltages, the pull downdevice draws too much current and drives the common source and thedrains of both transistors to ground before the digit lines can reachthe proper voltages.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art fora memory device which functions properly over a wide range of powersupply voltage.

SUMMARY OF THE INVENTION

A memory device with a sense amplifier is described which operatesacceptably over a wide range of power supply voltages. In oneembodiment, the present invention describes a circuit for a senseamplifier for use with a memory device that is operable over variedpower supply voltages. The circuit includes two devices that arecontrolled by a selector. The first device drives the sense amplifierwith a first current level. The second device drives the sense amplifierwith a second current level, different from the first current level. Theselector is coupled to the first and second devices so as to selectivelycouple devices to the sense amplifier based on a power supply voltage ofthe memory device.

In one embodiment the devices are n-channel MOS transistors thatfunction as pull-down devices for an n-sense amplifier. The pull-downdevices may have different widths so as to drive the sense amplifierwith different current levels. Alternatively, the pull-down devices mayhave similar widths and be selectively coupled alone or in combinationto the sense amplifier to produce an acceptable current to drive thesense amplifier.

In further embodiments, the selector comprises a Schmitt trigger thatproduces a signal that selectively activates the devices based on thepower supply voltage. The Schmitt trigger receives a signal that isproportional to the supply voltage and compares the signal with athreshold value of the Schmitt trigger. In further embodiments, theselector circuit comprises logic gates coupled between the Schmitttrigger and the first and second devices so as to produce signals toselectively activate the devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an illustrative embodiment of thepresent invention; and

FIG. 2 is a schematic diagram of an embodiment of a selector logiccircuit for use with the embodiment of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings which form a part hereof,and in which is shown by way of illustration specific preferredembodiments in which the inventions may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that logical, mechanical and electricalchanges may be made without departing from the spirit and scope of thepresent inventions. The following detailed description is, therefore,not to be taken in a limiting sense.

FIG. 1 is a schematic diagram of a memory device, indicated generally at10. Device 10 uses dual or folded digit lines to transfer data to andfrom memory cells via input/output (I/O) port 12. Device 10 includes rowlines 16, digit lines 18, and digit complement lines 20. A memory cell22 is coupled to each row 16 at the intersection with either a digitline 18 or a digit complement line 20. Sense amplifiers 14 are coupledto a corresponding pair of digit line 18 and digit complement line 20.The operation of device 10 is not tied to the folded digit lineconfiguration shown in FIG. 1. Device 10 may, alternatively, use an opendigit line or other appropriate configuration for the array of memorycells that can be accessed through sense amplifiers 14.

Device 10 further includes circuitry that selects a memory cell 22 fromdevice 10 to receive input or provide output to an external device suchas a microprocessor (not shown) at I/O port 12. Address buffers 24receive an address at input port 26 from the external device. Addressbuffers 24 are coupled to row decoder 28 and column decoder 30. Columndecoder 30 includes input-output circuitry that is coupled to theexternal device at I/O port 12. Row decoder 28 is coupled to rows 16.Column decoder 30 is coupled to digit lines 18 and digit complementlines 20.

Each sense amplifier 14 includes first and second n-channel MOStransistors 34 and 36 in a cross coupled configuration so as to form abasic flip-flop. A source of transistor 34 is coupled to a source oftransistor 36 at node 38. A gate of transistor 34 is coupled to digitcomplement line 20 and a gate of transistor 36 is coupled to digit line18. A drain of transistor 34 is coupled to digit line 18. A drain oftransistor 36 is coupled to digit complement line 20. First and secondpull down devices 40 and 42 are coupled to node 38 so as to drive senseamplifier 14. First and second pull-down devices 40 and 42 comprisen-channel MOS transistors each having a drain coupled to node 38 and asource coupled to ground. Selector circuit 44 is coupled to a gate ofeach of pull-down devices 40 and 42 so as to selectively activate eitherpull-down device 40 or pull-down device 42. Advantageously, pull-downdevices 40 and 42 have different widths so as to provide acceptablepull-down capabilities for sense amplifier 14 at different power supplyvoltages.

In operation, device 10 receives an address of a selected cell ataddress buffers 24. Address buffers 24 identify a row 16 of a selectedcell 22 to row decoder 28. Row decoder 28 provides a voltage on line 16to activate access transistors 30 of each cell 22 in the selected row16. The charge on the capacitor 32 is coupled to one of the digit lines18 or digit complement lines 20. Sense amplifier 14 senses a slightdifference between the voltage on digit line 18 and the voltage on digitcomplement line 20 of the selected cell 22 and drives digit line 18 anddigit complement line 20 to the value of the power supply rails.

Sense amplifier 14 assures proper operation over a wide range of powersupply voltages by selectively coupling either first or second pull-downdevices 40 and 42 to node 38. At a high power supply voltage, selector44 couples pull-down device 42 to node 38 when data is ready to bewritten to or read from cell 22. Pull-down device 42 is fabricated witha width that is less than the width of pull-down device 40 such thatpull-down device 42 draws a sufficiently low current that allowstransistors 34 and 36 sufficient time to determine the content of theselected cell 22.

At low power supply voltage, selector 44 couples pull-down device 40 tonode 38 when data is ready to be written to or read from cell 22.Pull-down device 40 is fabricated with a width that is, for example,approximately twice as big as pull-down device 42. Thus, pull-downdevice 40 draws a sufficiently high current so as to assure thattransistors 34 and 36 operate fast enough to accurately write data to orread data from a selected cell 22. Therefore, by using two pull-downdevices, a manufacturer can produce memory devices that functionproperly over a wider range of power supply voltage.

FIG. 2 is a block diagram of an illustrative embodiment of a selector,indicated generally at 44a. Selector 44a selects between first andsecond pull down devices 40 and 42 of sense amplifier 14 based on thepower supply voltage. Selector 44a comprises a Schmitt trigger 46 thatreceives a voltage that is proportional to the power supply voltage;labelled as V_(CC) /N. This voltage may be supplied by either anexisting node of a memory device or generated with circuitry as known toa person of ordinary skill in the art. In some memory devices, a nodeexists that is established at one-half of the power supply. This nodemay be advantageously used as an input to the Schmitt trigger. Selector44a further includes first and second NOR-gates 48 and 50 and inverter52. The output of schmitt trigger 46 is coupled to a first input ofNOR-gate 48 and an input of inverter 52. The output of inverter 52 iscoupled to a first input of NOR-gate 50. Finally, a latch signal, NLAT*,is coupled to a second input of both NOR-gate 48 and NOR-gate 50.

In operation, selector 44a selects first pull-down device 40 when thepower supply voltage is low and selects second pull-down device 42 whenthe power supply voltage is high. Schmitt trigger 46 receives a signalthat is proportional to the power supply voltage. The threshold ofSchmitt trigger 44a and its input are selected such that for high powersupply voltages, Schmitt trigger 46 outputs a high logic value and forlow power supply voltages, Schmitt trigger 46 outputs a low logic value.

When the input to Schmitt trigger 46 exceeds a threshold value, Schmitttrigger 46 outputs a high logic value to inverter 52 which produces alow logic value. In response, NOR-gate 50 produces a high logic outputwhen the latch signal goes low which turns on second pull-down device42. Thus, selector 44a selects second pull-down device 42 when the powersupply voltage is high.

When the input to Schmitt trigger 46 does not exceed the thresholdvalue, schmitt trigger 46 outputs a low logic value. In response,NOR-gate 48 produces a high logic output when the latch signal goes lowwhich turns on first pull down device 40. Thus, selector 44a selectsfirst pull-down device 40 when the power supply voltage is low. Due tothe differing widths of first and second pull down devices 40 and 42,selector 44a allows sense amplifier 14 to function properly over a widerange of power supply voltage by activating a pull down device that willdraw an appropriate current from sense amplifier 14.

Conclusion

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiment shown. This application isintended to cover any adaptations or variations of the presentinvention. For example, the sense amplifier is not limited to use with afolded digit line configuration. For example, open digit line or otherconventional configurations may be used. Further, selector 44 can bemodified with different logic circuitry so long as the circuitry allowsselector 44 to select between the pull-down devices based on the powersupply voltage.

The arrangement of pull down devices 40 and 42 can be varied withoutdeparting from the spirit and scope of the invention. For example,devices 40 and 42 can be fabricated in various proportions so long asthe devices provide acceptable current levels over the specified powersupply voltage range. For example, first and second pull-down devices 40and 42 can be replaced by two transistor with similar widths. In thiscase, selector 44 coupled one of the transistors to drive the senseamplifier when the power supply provides a high voltage. When the powersupply voltage is low, the selector couples both of the transistors todrive the sense amplifier.

Sense amplifier 14 of FIG. 1 is shown as an n-sense amplifier. As isknown in the art, sense amplifier 14 can also include a p-senseamplifier such that the n-sense and p-sense amplifiers are fired in turnto read from or write to a memory cell. The n-sense amplifier drives adigit line 18 or digit complement line 20 to a the low power supply railand the p-sense amplifier drives the other of digit line 18 and digitcomplement line 20 to the value of the high power supply rail. Inconventional operation, either the p-sense amplifier or the n-senseamplifier is fired first. A further embodiment of the present inventionincludes a p-sense amplifier first and second pull-up devices that areselectively coupled to drive the p-sense amplifier based on the powersupply voltage. Thus, the p-sense amplifier can be fired before then-sense amplifier and the sense amplifier will operate acceptably over awide range of power supply voltages.

What is claimed is:
 1. A method of driving a sense amplifier comprisingthe steps of:driving the sense amplifier through a first element;sensing a power supply voltage level with a Schmitt Trigger; andselectively driving the sense amplifier through a second element as afunction of the sensed power supply voltage level.
 2. The method ofclaim 1, wherein driving the sense amplifier through a first elementcomprises driving the sense amplifier through an n-channel transistor.3. The method of claim 1, wherein sensing the power supply voltagecomprises sensing when the power supply voltage exceeds a thresholdvoltage.
 4. The method of claim 1, wherein selectively driving the senseamplifier through a second element as a function of the sensed powersupply voltage level comprises driving the sense amplifier with thesecond element when the power supply voltage is above a threshold level.5. The method of claim 1, wherein selectively driving the senseamplifier through a second element as a function of the sensed powersupply voltage level comprises driving the sense amplifier with thefirst and second elements when the power supply voltage is below athreshold level.
 6. A driver for a sense amplifier comprising:a firstdriver, with a first n-channel transistor having a first width coupledto the sense amplifier; and a second driver, with a second n-channeltransistor having a second, different, width coupled to the senseamplifier, the second driver being responsive to a power supply voltagelevel.
 7. The driver of claim 6, and further including a selectorcircuit coupled to the first and second drivers.
 8. The driver of claim6, wherein the selector circuit includes a Schmitt trigger that comparesa voltage level proportional to the supply voltage with a referencevoltage.
 9. The driver of claim 6, wherein the Schmitt trigger producesa signal that selects among the first and second drivers to provide aselected current level for the sense amplifier.
 10. The driver of claim8, wherein the Schmitt trigger is coupled to a logic circuit thatcontrols the first and second drivers.
 11. A driver for a senseamplifier comprising:a first driver coupled to the sense amplifier; asecond driver, coupled to the sense amplifier, the second driver beingresponsive to a power supply voltage level; and a selector circuit, witha Schmitt trigger that compares a voltage level proportional to thesupply voltage with a reference voltage, coupled to the first and seconddrivers.
 12. The driver of claim 11, wherein the first and seconddrivers comprise first and second n-channel transistors.
 13. The driverof claim 12, wherein the first and second n-channel transistors havesubstantially the same width.
 14. The driver of claim 11, and furtherincluding a selector circuit coupled to the first and second drivers.15. The driver of claim 14, wherein the selector circuit includes aSchmitt trigger that compares a voltage level proportional to the supplyvoltage with a reference voltage.
 16. The driver of claim 15, whereinthe Schmitt trigger produces a signal that selects among the first andsecond drivers to provide a selected current level for the senseamplifier.
 17. The driver of claim 15, wherein the Schmitt trigger iscoupled to a logic circuit that controls the first and second drivers.